Hydraulic hoisting unit and control system therefor



7 Sheets-Sheet 1 original Filed sept. 22, 1933 AHrDRAUL-m HOISTING UNIT AND CONTROL SYSTEM THEREFOR Aug. 29, 1939.

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INVNTOR. 7%"3/ kers BY /uw 12h44, LLC fPwf! ATTQRNEYS Aug. 29, 1939.

H. F. VICKERS HYDRAULIC HOISTING UNvIT AND CONTROL 'SYSTEM THEREFOR 7 Sheets-Sheet. 2 .Y

Original Filed Sept. 22, 1935 ngvENToR. 5yd-wy 2 Vickers M; M, 9.44. d

ATTORNEYS Aug. 29, 1939.

H. yF. vlcKERs HYDRAULILC HOISTING UNIT AND CONTROL SYSTEM THEREFOR Original Filed Sept. 22,4 1935 7 Sheets-Sheet 4 y ewig-El? Mm @uw w ATToRNEYa Aug. 29, 1939. 2,171,204

' HYDRAULIC HoIsTING UNIT AND CONTROL SYSTEM T HER'ENR H. F. VICKERS original Filed sept. 22.*.193'3 ,7 sheets-sheet 5 1NVENTOR f Zum Ai,

- 15m; @NYM 4g-4 ATTORNEYS Aug. 29, 1939. H. F. VICKI-:Rs 2,171,204

HYDRAULIC HOISTING UNIT AND CONTROL SYSTEM THREFOR l( Original Filed Spt. 22. V193.3 '7 Sheets-Sheet 6 INVE'NTOR, I' Vickers ,00 zwv ATTORNEY@` Aug. 29, 1939. H. F. vlcKERs 2,171,204

HYDRAULI HOISTING UNIT AND CONTROL SYSTEM THEREF'OR Original Filed Sept. 2 2, 1933 7 Sheets-Sheet 7' 1N VEAVF OR.

. ATTORNEYS latented Aug. 29, 1939 @PATENT OFI-lcs HYDRAUIJCI-nols'rmo UNIT AND CONTROL SYSTEM THEREFOR .'Harry F. Vickers, Detroit, Mich., assignor 'to Vickers, Incorporated, Detroit, Mich., a corporation o Michigan Original application September 22, 1933, Serial No. 690,630. Divided and this application cember 15, 1934, Serial No. 757,619.

18 Claims.

This invention relates to a hydraulic hoisting unit and control system therefor, and has to do particularly with improved hoisting and lowering means in combination with positive and exv tremely safe controls together with improved accelerating and decelerating means relative to starting and stopping position.

This application isa division of my 4copending application Serial No. 690,630, filed September 22, 1933.

One of the important features of my invention has to do with a novel directional control valve and associated structure, so designed in combination with thevalve seat `as to, ,provlde a definite rate of now and a resulting fixed speed rate of hoisting or lowering. An important element of this feature resides in the fact that the taper on' the valve determines the volume flow and` in combination with-associated structure determines a xed rate of movement, and a xed lowering' speed rate regardless of load.

Another feature of the invention-has to d0 with an automatic volume control Working in combination lwith the directional control to assist in providing fora definite pressure drop through the orifice formed bythe directional control`mei'nber, said automatic volume control also serving as a maximum pressure controlvv whereby tosprovide maximum acceleration dur- 30 ing the hoisting stroke.

A further feature of the present invention has to do with a mechanical safety feature in the form of brakes operating in combination with 1- the control mechanism whereby to insure p'osi- 35 power release before the brake is set, and without any appreciable wearing of the braking surface material. i

A still further feature has to do with.a combined pump unit operating in combination' with the control unit and the hoist motor and including a small volume pump continuously operable to provide positive pressure for the control -mechanism and a large, volume pump operable only during the hoisting step, said two pumps being interconnected through a combined relief land balancing means, responsive to an increase in demand upon the large volume pump for allowing free flow from the small volume pump to the large volume pump line; resulting in the output of `the small volume pump being delivered to do useful work along with theoutput 'of the large pump. Y l

Other novel features including the means for relieving -the large'pump o'f operating pressure,

tive locking at the proper time and to preventl (ci. isz-3) Fig. 2 is a plan view mostly in 10 section of the ,combined pump unit and particularly illustrating Athe combined relief and control valve.

Fig. 34 is a side elevation of the hoist unit and control mechanism.

Figs. 4 and 5 represent an end view and a partially cut away end view, respectively, of the vhoist and control mechanism, and particularly showing the braking mechanism and control therefor. .j

Fig. 6 is a sectional .view taken on line 6-6 of Fig. 3 and showing particularly the volume controlan'd directional control units.

Fig. 7 is a plan view of the structure shown in Figs. 3 to 5.

Fig. 8 is a sectional view taken on line 8-8 of` Fig. 7. l

Fig. 9 is afragmentary plan view taken On lines` 9-9 of Fig. 8 and being directed particularly to the balanced gear housing of the hoist unit.

Fig. 10 is a vertical sectional view taken on line llllll of Fig. '7.

y As best shownv Yin Fig. 1, the hoisting unit and control 4system includea two section pump unit generally indicated as at 2, a'control unit generally designated 3 including an'\auxiliary hand operated four way control unit or pilot valve 3a of standard type, a motor and pump unit for the hoist, generally designated 4, and a braking or auxiliary hydraulically Aoperated unit 5.

Y Referring particularly to the two section pump unit, as best illustrated in Fig. 2 and designated X, this unit consists of a large volume pump unit generally designatedfS and located adjacent the shaft'coupling l which connects the pump unit lwith the initial sourceof power; and aysmall volume pump generally designated 8. Both pump sections are preferably positive displacement type pumps, the large pump supplying pressure through the line 9 for directly controlling the hoisting unit and the small "pump supplying pressure through the line I0 to control the brake and controlV valves (see Fig. 1)

' As best shown in Fig. 2, both pump sectionsG and 8 are connected to a manifold Y having a common inlet ll and branch conduits I2 and I3,

respectively. The liquid under pressure from section 6 passes through the outlet 9a of the connection Z and, Vas will be later described, Athe large pump 6 is relieved of operating pressure -except during the actual hoisting operation. This results in a considerable power saving, as otherwise thelsingle large volume pump would have to operate at full pressure at all times. Oil also ows through the small pump unit 8 into the chamber I4, which chamber also surrounds a valve I5. The valve I5 is provided with a-small piston I6, and the iiow from the chamber I4, through the conduit Illa, t'o the right hand side of the piston' I6 is by means of a threaded resistance .passage 'I5 formed between the parts I1 and I8, the chamber to the left of the piston I6 being vented through the conduit I9 to make the pressure in the chamber to the right of the piston, designated 20, eiective. In practice there is no communication between the chamber at the left of the piston I6 and the conduit I9, there is enough clearance between the ends of the parts I1 and I8 to assure`drainage into the conduit I9.

As the pump 8 is continuously operated, the oil under pressure flows through the passageway and chamber I4 through the resist-ance passage movement of the valve.

lso

and into the chamber 20, exerting thrust to the left which tends to overcome an adjustable tension spring 2I. `By overcoming the spring 2l, the pressure inthe chamber causes the valve I5 to move slightly to the left and take up a position whereby oil is bypassed into the chamber 22 which leads to the main operating passageway 9a, in just sufficient amount to maintain a bal-- ance between the pressure in the chamber and the resistance of the spring 2I,the oil being restricted in its iiow from the chamber I4 to the chamber 20 a proper amount to tend to maint-ain this balance; or, in other words, this restriction is suiiicient to prevent chattering and sudden in valve.l5`functioning as a relief-Avalve during the time that the pressure in the chamber 22, and of course, the'main outlet passage 9a, is below the adjusted pressure of the spring 2I, with the result that a predetermined pressure is maintained in passageway and chamber I4 at all times.. so asv to deinitely assure operating pressure for the control parts of .the system.

l When this control pressure which ows from the chamber I4 through the line I0 to the unit' l3d (see Fig. 1') is utilized to operate the control unit to hoist, the pressure in the outlet 9a and chamber 22 rises, causing a 'back' pressure against I4.rten`di ngctolfurther` overcome the spring 2| and open the valve-I5 from its normal correct position. ln the function of this circuit we may assume an adjusted operating pressure o ve v hundred poundsl per square inch on the spring 2l and a nal operating pressure of seven hun-v small pump B'being delivered to do useful workY along with the output of the large. pump 3, The result and operation of this circuit under This condition results and at predetermined intervals, and at the same time maintaining a somewhat lower operating pressure in the chamber I4 to assure a constant pressure supply in the line I0, which leads from the chamber I4, to the pilot or operating valve 3a and to the braking unit 5.

Referring particularly to the control unit 3, as best shown in Figs. l and 6, it will be seen that the large volume pump assisted by the small vol-v ume pump supplies pressure through the line 9 and into the chamber 23 of the main control valve unit 3. This chamber 23 surrounds an automatic volume control valve 24 and also extends to a position where it surrounds and communicates with a directional control valve 25. A conduit 26 connects the control valve 3a with the left hand side of the directional control valve 25 so that, by movement of the control valve 3a, pressure is admitted from the line I0 into the conduit 26 to shift the directional valve 25 to the right. This action permits oil to flow from the line 9, through chamber 23, into the chamber 21,`which chamber is connected through to passage 28 which leads to the motor 4.-

The valve 24 is provided with a piston valve having a piston portion 29 which is provided with ends or piston rods on each side of equal diameter. The piston 29 is normally urged to the right by `a spring 30 of predetermined tension' to normally prevent the flow of oil under pressure from the chamber 23 into the outlet passage 3I which leads to the return tank R. It will be seen that the valve 24 will remain on its seat as long as the pressure in' the chamber 23, communicated through passageway 32 to a chamber 33 at the left hand side of the piston 29, plus the predetermined thrust of the spring 30, is sufcient to force all the pump output through the orice,

which is formed by the wall Aof the chamber 21 and the tapered portion 34 of the valve 25i when ing in communication with the exhaust chamber 60 when the valve 2'5 is moved to the left, as will be later explained. By way of example, it will be seen that if"the spring 30 back of the piston 29 has a thrust equivalent to-.twenty pounds per square inch, such piston will maintain a, closed position relative to the passage 3|, until such time `s thevolume f liquid passing through the chamber 23 requires more than twenty pounds per 'lsquare inch to force this volume of liquid throughthe clearance between the wall ofthe chamber 21 and the vtapered portion 34-of the valve. It will thus be seen that the valve 24 will provide a definite pressure drop across an orifice of given size, resultingdn a definite rate of oy lthrough this orifice. It will thus be seenthat the amount of taper 34 on the valve 25 becomes a method of xing the speed rate of hoisting.

The valve 25 is provided with a cam 35 which normally contacts with a cam follower 36 when such valve 25 is in neutral position Movement or locking unit consists of supported pivoted arms 40 and, 4|. These arms and the brake shoe portions 42V carried thereby lare `normally forced against the brake drum 43 because of springs 44 so that normally the arms 40 and 4| grasp and hold thefbrake drum mechanically. The cylinder 39 is positioned intermediate the arms 410 and 4| and carries oppositely acting pistons 45, with the result that substantially simultaneously with the` raising, of `the valve 31, the pressure from the line |a.will be communicated through the line 38, into the cylinder 39 urging the pistons `45 outwardly to contact with and release the brake shoes 42.

It is desirable during non-hoisting operation to allow the pump oil to flow from the chamber 23l through the exhaust passage 3| back to the re.v

- turn tank or reservoir R. This is accomplished by normallyl venting the passage 33, whichis at the rear of the piston 29 to the atmosphere, so

4that any pressure in the chamber 23 exceedingV the resistance of the spring 30 will cause the valve 24 to move tojthe left. A valve '31a being normally downwardly spring pressed, will, whenN the' plunger,` '31 is forced downwardly, dueto the di; rectional valve 25 returning to neutral position,

connect thechamber 33 with the atmosphere or,

more s pecically speaking, the' return line, by means of the conduit 46, conduit46a connecting the cylinders of .valves 31a and 31, and conduit 46b connecting the cylinder of valve 31 with the reliefor drain conduit 48;i

`During the hoisting operation,the brakeshoes being released by the application of pressure to the line 3,8, it will be seen that ythis movement also supplies pressure to the bottom of the valve 31a, overcoming the spring and moving this valve 31a upwardly andthus closing the exhaust vent so that during this period4 the chamber 33 is open to the effectifepressure from theline 32. It will be seen that this construction and circuit aref fective to positively reli'egethe large pump 6 from' loperating pressure except whenhoisting. VThe hoisting unit 4, as shown in Fig.' 1, and better illustrated in Figs. 7 to 19, embodies a'balanced gear motor of the general construction described in my Patent No. 1,937,967A of- November 28, 1933.

Teeth subjected to pressure in the pressure cham- -ber at one side of the motor are balanced by pressure chambers 28a. andthe teeth on the other side, namely those on the exhaust side being relievedby balancing chambers 3|a.

When the valve 24 is closed and the valve 25 movei to the right te permit the new of liquid under pressure intothe conduit 28, the hoisting motor, generally designated 4, is actuated. to roltate the shaft and the drum 50. As best shownk in Figs. 1, 3 and 10, the other end of this shaft 5| carries thebrake member 4,3 and also'a traveling nut 52, connected with arms .54 pivoted as et sa to transmit motion teA a. .sliding sleeve is'.

Thissleeve 5 5 surrounds an ltensign of the valve 25. A threaded portion 52a. which \oper ates the traveling nut 52 is provided with threads of such pitch, proportioned in' accordance with valve 6| formed integrally with a piston 62.

the leverage of the arms 54 and the hoistingdistance or height so that when the'member being hoisted approaches the extreme limit of such hoisting distance, the sliding sleeve 55 contacts with adjustable nuts 56 'so that during the last portion of the movement, -thesliding sleeve 55 will move the valve 25 towards central or Aneutral position to gradually stop the iiow of liquid to the chamber 21 from the chamber 23.D This same movement depresses the plunger 31 and permits the valve 31a to lower to exhaust theu chamber 33 to permit movement of the piston 29 to the left. Thus stopping movement of the motor 4 and also'simultaneously cutting ofi the pressure from the line |0a and evacuating the chamber 39, allow the brake springs to apply brake pressure and hold the hoist at the place where it has just stopped, due to movement of the directional control valve 25. The tendency to hoist by the motor 4 is not completely removed l until after the brakes are applied; this is accom.-

plished by using a. spring backed valve 31a of less strength than the back pressure built up by the sudden exhausting of oil from the cylinder 39 by the springs 44.

Movement of the control valve 3a in the opposite direction effects lowering of the elevator or .hoist unit in that pressure from the small pump 8 is conducted through the line 51 to the chamber at the right of thevalve 25 (see Fig. 1) to shift the valve to the left. This movement relieves the pressure in line 26 and also results in applyingof pressure to the line 38 to operate the pistons 45 and to relieve the brake shoes as previously described.

During this lowering action, the motor 4 will operate as a pump and, referring to Figs." 1 and 6, it will be seen that oil in the passage 28 will flow through a chamber 58 to a. passageway 59 and due to the position of the valve 25, when moved-to the left, will pass into` passageway 60.

connected to the reservoir. 'I'he connection be-r tween the chambers 58 and 59 is controlled by a This valve 6| is normally urged to the right, as viewed in Fig. 6, by a spring 63 of predetermined tension.

As best shown in Fig. 1, chambers 64 and 65 areconnected to passageway 59 by .means of conduit 64a, thus permitting any pressure in passagewayA 59 to tend to overcome the tension of the spring 63, and in turn causing the valve 6| to restrict the passage of oil from the chamber 58 to the passageway59. A An unrestricted passageway 66 directly connects the chambersY and 65a (as shown in dotted lines in Fig. 1 thus permitting the pressure in passageway' or chamber 60 to aid the spring 63 in resisting the effect of pressure in the passageway 59. In operation, during the lower'- =ing of' the hoist, the piston 62 and valve 6| will take up a. position wherein the difference inpressure existing in chamber 59'and chamber 69 will be equivalent to the resistance of the spring 63 :mainstfluid pressure in chambers 64 and 65.

A'tapered surface 34a at the opposite end of .the valve from the tapered surface 34 will, when the valve 25 moves to-the left, act as-an orice on throttling means between the chambers 59 and 60. Therefore, it will seen that the balancing of the piston 62 and valve 6| determines a pres sure drop acrossl the orice formed by the degree of taper between the surface 34a and thewall of chamber 59 resulting in a xed flow rate which in turn results in a xeddescending rate for the elevator or hoist.

It will thus be seen that dlerent rates of hoisting and/or lowering can be obtained by the simple step -oi placing a new valve member 25 in the control unit having a taper 34 and 34a predetermined in accordance with the particular hoist for which it is to be used; it being under-v stood, of course, that the degree of taper on 34 and 34a may be different, dependent upon the relative rate of hoisting, lowering, and deceleration desired in the particular job.

Descent of the hoist or elevator continues until the traveling nut 52, moving in the opposite direction, actuates the sleeve 55 until contact is made with the adjusting nuts 61 and thus moving valve 25 toward neutral position, and gradually decelerating and then stopping the descent of the hoist or elevator. Cam 35 again lowers the plunger 31 to exhaust cylinder 39 allowing the brakes to .be applied to hold the hoist in its stopped or locked position.

The conduit 32, see Fig. l, has a double function in that it not only acts in assisting to maintain a constant head across the orice formed by the surface 34 during hoisting, but also functions in combination with a valve 68 to provide a maxlmum pressure control of the ypump supply during hoisting, thereby providing maximum acceleration and control during the hoisting stroke. Referring more particularly to Fig. 6, the valve 68 includes a spring 69, thestrength of which may be relatively adjusted by the mechanism 10 so that the spring 69 holds the valve member 'H on its seat until the pressure in the chamber 33 exceeds the resistance of the valve 'H and.- spring 69. If the pressure in chamber 33 should develop sufficiently to unseat the valve 1 I, liquid will' pass by said valve and through a drilled hole in the valve member '24 and piston 29, with the result that the valv'e 24 and piston 29 will rapidly unseat to bypass the surplus oil to the tank. The valve 24 and piston 29 in this connectionA act as a relief valve similar to the relief valve disclosed in my Patent No. 2,043,453 issued June 9, 1936, inasmuch as the passageway 32 is formed smaller than the passageway 'l2 leading from the chamber 33 to the valve 1|. It is therefore obvious that the valve 24 acts as a non-chattering relief valve and that any increase in pressure in t-he chamber 23 beyond the maximum pressure seating of the valve 1I, which controls the pressure in the chamber 33, will cause the valve 24 to unseat, allowing excess oil to pass from the chamber 23 to chamber 3l and to the retur/nv tank. Thus this valve acts both asa means for maintaining constant head across the fixed orifice andy also as a relief valve for controlling maximumv Pressure. l,

When the valve `'25 moves to the left, the tapered portion 34a will uncover the extreme right hand'end of passageway 32 and open same to the exhaust conduit 60, unseating the valve.24.

Attention is called to the important point that the combined relief andunloading valve permits -the small volume pump to operate at the same pressure as the large volume pump, otherwise it would operate at a higher` pressure during hoisting.

It will be seen that the degree of taper 3l What I claim'is:

1. A hydraulic actuating system comprising a source of pressure supply including a relatively large volume'pump and a relatively small volume pump, a main hydraulically operated unit actu- A large volume pump and a relatively small volume pump, a main hydrau'lically operated unit actuated by both pumps, an auxiliary hydraulically and mechanically operated unit cooperating with said first named unit and hydraulically actuated solely by said small pump, and means for relieving said large pump when said rst named' unit 4 is inoperative. l

4. A hydraulic actuating system comprising a source of pressure supply,l a hydraulic motor unit connected to means to be actuated, a directional vcontrol unit hydraulically Iconnected to said source of supply and said vmotor unit, mechanical locking means for said motor unit, means controlling the `application and'v release of said locking means, and means movable in unison with aimovable part of said directional control unit for actuating said locking control means.

5. A hydraulic actuating system comprising la 'source of pressure supply, a hydraulic motor unit connected to means to, be actuated, a combined hydraulic and mechanical locking means for the movable element of said motor unit, valve means hydraulically connected to said source of supply and said locking means, and means actuated by the movement of said movableelement of the motor. unit to a ,predetermined point for effecting application of said mechanical lockingmeans.

6. A hydraulic actuating system comprising a source of pressure supply, a hydraulic motor unit connected to means to be actuated, a combined hydraulic and mechanical locking means for the movable element of said motor unit, valve means hydraulically connected to said source of supply and said locking means, and means actuated by the movement of said movable "element of the motor unit to a predetermined point for effecting -application of said mechanical locking means,

said last named means and saidl valve cooperating to effect by-passingv of said source of supply.

7. A hydraulic actuating system comprising a source of pressure supply, a hydraulic motor unit connected to means to be actuated, a combined hydraulic and mechanical locking means for the movable element'of said motor unitf valve means hydraulically connected to said source of 'supply and said locking means, and means actuated by the movement of said movable element of the motor unit toa predetermined point for eilecting application-.of said mechanical locking means, said lastv named meansA and said valve cooperating to eiect by-passing of .said source of supply but soarranged as to insure maintenance ci pressure upon the motor unit until said locking iniieans has been moved to positive locking'posi- 8. A hydraulic actuating system comprising a source of pressure supply,.a hydraulic motor unit connected to means to be actuated initwo directions, directional control means, oriiices through which liquid flows to and from said unit, means `responsive to the position of said control means for eiect-ing .a predetermined pressure dropacross said orices whereby to control the rate of movement of said unit in either direction, mechanically applied locking means for the movable element of said unit,'and means responsive to movement of said directional control means for hydraulically releasing said locking means. 9. A hydraulic actuating system comprising a source of pressure supply, a hydraulic motor unit connected to means to be actuated, locking means for the movable element of said unit, directional control means, means actuated by the movement of said movable element to a predetermined position for moving said`contro1` means towards neutral position to stop the ow of liquid and stop said movable element, and means responsive to said movernent'ofsaid control means for effecting application of ,said locking means to -hoid the movable element in its arrestedv position.

10. A hydraulic actuating system comprising a source of pressure supply, a hydraulic motor unit connected tomeans to be actuated, locking means for the movable-element of -said unit, a main hy-l draulic control unit positioned between the source of supply and said motor unit and including speed control means for controlling the rate of movement of said movable element, and auxiliary control means hydraulically connected to'said source 35 of supply and said locking means and actuated by the movement .of said. movable element to a predeterimned position for by-passing said source of supply.` s

11. A hydraulic actuating system comprising 4o a source -of pressure supply,. a hydraulic motor unit connected to'means to be actuated in two directions, directional control means, orifices' through which liquid ilows to and from said unit, means responsive to the position of said control me'ans for effecting a predetermined pres. sure drop across said orifices whereby-to control the rate of movement of said unit in either di rection, and'means forming a part of the hydrauliccircuit for locking said rst named means .5o at any point in its path' of movement.

12. A hydraulic actuating system comprising a source of pressure supply, a hydraulic motor unit connected to means to be actuated in two directions, `directional control means, orices through 5'5' which liquid iiows to and from saidv unit, means responsiveto the position of said control means for effecting a predetermined pressure'drop acrossV said orifices whereby to'control the rate of .movement of said unit in l'either direction, means responsive to the movement of the movable element of said motor unit for relieving said .source of supply when said ,movable element reaches a predetermined position, and means forming a l partei' the hydraulic circuit for locking saidrst named means at any point in its path of movement. v l I 1 3, `A hydraulic actuating system comprising a vsourde of pressure supply,.a hydraulic power unit including a movable element, the movable ele- '70 ment beingconnected to means to be actuated,

a pressurev relief unit positioned between the source ofsupply and said -power unit, saidpressure relief 4unitin'cluding a v'alve'member adapted in closed position t o -direct the iiow of iuid pres- 'Il sure tonthe power unit and in open position to byduits connecting the pump and motor, of a relief supply line.

.diiferential onf saidpiston for opening the valve pass' the ow of iiuid pressure back to the tank, means for normally maintaining said valvein 4closed position up to a predetermined pressure,

and means actuated by. the movement of the power unit to a. predetermined position in its fnorg5y mal path, of movement andv hydraulically connected with said pressure relief unit to control the opening of said valve member to relieve the source of pressure supply, independently of the pressure of said supply; l

14. A hydraulic actuating system comprising a source of pressure' supply, a hydraulic power unit including a movable element, the movable element being connected to means to be actuated,

a pressure relief uni-t positioned between. the 15 .source of supply and said power unit, said pressure relief unit including a valve member adapted in closed position to direct the ow of fluid pressure tothe power. unit and in open position to bypass the iiow of iuid pressure back to the tank, means for normally maintaining said valve in' closed position up to a predetermined pressure,

and means hydraulically connected withl said pressure relief unit to control the opening of said valve member to relieve the source of pressure supplyfindependently ofthe pressure -of said Asupply, I

l5. In a hydraulic power transmission system the combination with a pump, a'uid motor driven by the pump, and supply and return convalve in the supply line adapted at times to con` nect the supply conduit with the lreturning conduit for by-passing the delivery of the pump, said relief valve having an. operating piston whereby the valve may be opened or closed depending upon the iiuid pressures exerted on said piston, means responsive to excessive pressure in the supply line for causing a pressure diierential on said piston for opening the valve to prevent excessive pressure rises in'the supply line, am. additional selectively operable means for causing a pressure diierential on said piston to open -the valve for by-passing the pump to stop operation of .the motor independently of pressure in the 45 16.* In a hydraulic. power transmissionsystemthe combination with a pump, -ariiuid motor driven by the pump, and supply and return conduits vconnecting f the pump and motor of means normally controlling the supply of'iluid to the motor, a relief valve in the supply line adapted at times to connectthe^supply conduit with the return conduit for ily-passing the delivery of the pump, said relief v'alve having an operating piston whereby the valve may vbe `opened or'closed depending .upon the fluid pressures exerted -on said piston, means responsive to excessive pres-v sure in the supply line for causing a pressure in Athe supplyline, said additional means being connected. for operation concurrently with the normal/controlmeans.

, 17. In a hydraulic power transmission system the combination with a pump, a fluid motor driven bythe pump. and supply and return conduits connecting. the pump. and motor, of a relief valve in the supply line'adapted at times to connect the supply conduitl with the return conduit for-by-passing the delivery of the'puxpp. said u relief valve having an operating piston normally balanced with respectl to the supply line pressure, one side .of said piston communicating with the supply line through a restricted passage, a pressure responsive valve for venting the one side of the piston to open the valve whenever a. predetermined pressure is exceeded in the supply line, and

`'additional valve means for venting. the one side of the piston to open valve independently of supm ply line pressure.

Y18. In a hydraulic power transmission system' the combination with a pump, a uid motor driven by the pump, and supply and return conduits connecting the pump and motor of means normally controlling the supply of fluid to the motor, a relief valve in the supply line adapted at times to connect the supply conduit with the j return conduit for by-passing the delivery of the pump, said relief valve having an operating piston normally balanced with respect to the sup- 

